One of three nuclear localization signals of maize Activator (Ac) transposase overlaps the DNA-binding domain

The nuclear localization sequences (NLSs) of the Ac transposase (TPase) protein have been characterized by indirect immunofluorescence detection of TPase deletion derivatives and TPase/beta-glucuronidase (GUS) fusion proteins in transiently transfected Petunia cells. The TPase contains three NLSs near its amino-terminal end, NLS(44-62), NLS(159-178) and NLS(174-206), each of which is sufficient to redirect GUS to the nucleus. Deletion of the N-terminal 102 TPase residues including NLS(44-62) results in strongly reduced nuclear import of the truncated TPase. NLS(44-62) and NLS(159-178) are bipartite NLSs, whereas the structure of NLS(174-206) does not allow a classification into one of the three major NLS categories. NLS(174-206) overlaps with the basic DNA-binding domain of TPase. A substitution of two amino acids in this segment (His191-->Arg and Arg193-->His) results in a total loss of DNA-binding activity, but retains reduced NLS activity. Accordingly, the two functions can be separated. In addition, we show that a NLS-deficient 71 kDa TPase derivative is co-imported into the nucleus in the presence of wild-type TPase.